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[PATCH v2 2/3] parse-options: make OPT_ARGUMENT() more useful

From: Johannes Schindelin <johannes.schindelin@xxxxxx>

`OPT_ARGUMENT()` is intended to keep the specified long option in `argv`
and not to do anything else.

However, it would make a lot of sense for the caller to know whether
this option was seen at all or not. For example, we want to teach `git
difftool` to work outside of any Git worktree, but only when
`--no-index` was specified.

Note: nothing in Git uses OPT_ARGUMENT(). Even worse, looking through
the commit history, one can easily see that nothing even
ever used it, apart from the regression test.

So not only do we make `OPT_ARGUMENT()` more useful, we are also about
to introduce its first real user!

Signed-off-by: Johannes Schindelin <johannes.schindelin@xxxxxx>
 Documentation/technical/api-parse-options.txt | 4 +++-
 parse-options.c                               | 2 ++
 parse-options.h                               | 4 ++--
 t/helper/test-parse-options.c                 | 2 +-
 4 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/Documentation/technical/api-parse-options.txt b/Documentation/technical/api-parse-options.txt
index 2b036d7838..2e2e7c10c6 100644
--- a/Documentation/technical/api-parse-options.txt
+++ b/Documentation/technical/api-parse-options.txt
@@ -198,8 +198,10 @@ There are some macros to easily define options:
 	The filename will be prefixed by passing the filename along with
 	the prefix argument of `parse_options()` to `prefix_filename()`.
-`OPT_ARGUMENT(long, description)`::
+`OPT_ARGUMENT(long, &int_var, description)`::
 	Introduce a long-option argument that will be kept in `argv[]`.
+	If this option was seen, `int_var` will be set to one (except
+	if a `NULL` pointer was passed).
 `OPT_NUMBER_CALLBACK(&var, description, func_ptr)`::
 	Recognize numerical options like -123 and feed the integer as
diff --git a/parse-options.c b/parse-options.c
index cec74522e5..1d57802da0 100644
--- a/parse-options.c
+++ b/parse-options.c
@@ -286,6 +286,8 @@ static enum parse_opt_result parse_long_opt(
 					     optname(options, flags));
 			if (*rest)
+			if (options->value)
+				*(int *)options->value = options->defval;
 			p->out[p->cpidx++] = arg - 2;
 			return PARSE_OPT_DONE;
diff --git a/parse-options.h b/parse-options.h
index 7d83e2971d..c3d45ba1ac 100644
--- a/parse-options.h
+++ b/parse-options.h
@@ -138,8 +138,8 @@ struct option {
 	{ OPTION_CALLBACK, (s), (l), (v), (a), (h), (f), (cb) }
 #define OPT_END()                   { OPTION_END }
-#define OPT_ARGUMENT(l, h)          { OPTION_ARGUMENT, 0, (l), NULL, NULL, \
-				      (h), PARSE_OPT_NOARG}
+#define OPT_ARGUMENT(l, v, h)       { OPTION_ARGUMENT, 0, (l), (v), NULL, \
+				      (h), PARSE_OPT_NOARG, NULL, 1 }
 #define OPT_GROUP(h)                { OPTION_GROUP, 0, NULL, NULL, NULL, (h) }
 #define OPT_BIT(s, l, v, h, b)      OPT_BIT_F(s, l, v, h, b, 0)
 #define OPT_BITOP(s, l, v, h, set, clear) { OPTION_BITOP, (s), (l), (v), NULL, (h), \
diff --git a/t/helper/test-parse-options.c b/t/helper/test-parse-options.c
index cc88fba057..2232b2f79e 100644
--- a/t/helper/test-parse-options.c
+++ b/t/helper/test-parse-options.c
@@ -132,7 +132,7 @@ int cmd__parse_options(int argc, const char **argv)
 		OPT_NOOP_NOARG(0, "obsolete"),
 		OPT_STRING_LIST(0, "list", &list, "str", "add str to list"),
 		OPT_GROUP("Magic arguments"),
-		OPT_ARGUMENT("quux", "means --quux"),
+		OPT_ARGUMENT("quux", NULL, "means --quux"),
 		OPT_NUMBER_CALLBACK(&integer, "set integer to NUM",
 		{ OPTION_COUNTUP, '+', NULL, &boolean, NULL, "same as -b",